Recently, environmental considerations have dictated that effluent released to atmosphere contain very low levels of hazardous substances; national and international NOx emission regulations are becoming more stringent. NOx emissions are typically formed in the following manner. Fuel-related NOx are formed by the release of chemically bound nitrogen in fuels during the process of combustion. Thermal NOx is formed by maintaining a process stream containing molecular oxygen and nitrogen at elevated temperatures in or after the flame. The longer the period of contact or the higher the temperature, the greater the NOx formation. Most NOx formed by a process is thermal NOx. Prompt NOx is formed by atmospheric oxygen and nitrogen in the main combustion zone where the process is rich in free radicals. This emission can be as high as 30% of total, depending upon the concentration of radicals present.
Post-combustion units, such as that disclosed in U.S. Pat. No. 4,850,857 (WO 87/014 34), the disclosure of which is hereby incorporated by reference, have been used to oxidize process effluent. Such post-combustion units have many uses in industry, for example in the printing industry, where exhaust fumes may contain environmentally hazardous substances. The burners currently in use, however, emit NOx gases.
In order to ensure the viability of thermal oxidation as a volatile organic compound (VOC) control technique, lower NOx emissions burners must be developed.